Glycolaldehyde is an intermediate in many organic reactions and is particularly useful as an intermediate in the production of ethylene glycol through a catalytic hydrogenation process. Ethylene glycol is a valuable commercial chemical with a wide variety of uses, e.g., as a coolant and antifreeze, monomer for polyester production, solvent, and an intermediate for production of commercial chemicals.
Glycolaldehyde can be prepared by the rhodium catalyzed hydroformylation of formaldehyde under elevated temperatures and pressures. The hydroformylation reaction is typically carried out in an aprotic solvent which will dissolve polar materials. Suitable solvents include a wide variety and are exemplified by N,N-disubstituted amides, in which each hydrogen of the amido nitrogen is substituted by a hydrocarbon group, e.g., 1-methylpyrrolidin-2-one, N,N-dimethylacetamide, N,N-diethylacetamide, and N,N-dimethylformamide; nitriles, such as acetonitrile, benzonitrile, propionitrile and the like; cyclic ethers such as tetrahydrofuran, dioxane and tetrahydropyran; ethers such as diethyl ether, 1,2-dimethoxybenzene, alkyl ethers of alkylene glycols and polyalkylene glycols, e.g., methyl ethers of ethylene glycol, propylene glycol and di-, tri- and tetraethylene glycols; ketones such as acetone, methyl isobutyl ketone, and cyclohexanone; and esters, such as ethyl acetate, ethyl propionate and methyl laurate.
The separation of the glycolaldehyde product from the hydro-formylation reaction mixture in these processes, however, is often difficult and expensive because of the thermal sensitivity of the catalyst and reactivity of the the glycolaldehyde product. For example, the separation of glycolaldehyde from the hydroformylation catalyst solution by traditional distillation methods can be difficult because of the decomposition hydroformylation catalyst and the formation of glycolaldehyde byproducts. Various techniques for accomplishing this separation have been disclosed and include, for example, distillation (British Patent No.1,585,6004 and U.S. Pat. No.'s Re. 32,084 and 4,405,851), separation of glycolaldehyde as a distinct polar phase from the non-polar catalyst components (U.S. Pat. No.'s 4,496,781, 4,560,806, and 4,608,444), and extraction (U.S. Pat. No.'s 4,503,260; 4,405,814; 4,740,525; 4,477,685 and 4,382,148). These methods, however, are subject to various shortcomings. For example, distillation of the reaction mixture subjects the catalyst to conditions which can result in the decomposition of ligands and/or the irreversible precipitation of rhodium metal. Distillation, in some instances, can result in the formation of metallic mirrors on the surface of process equipment. In addition, distillation can greatly reduce the activity of the catalyst. The conversion of the glycolaldehyde product to an acetal or hemiacetal may improve its thermal sensitivity, but does not solve the problem of catalyst decomposition. This approach also suffers from the disadvantage that an additional hydrolysis step is required to obtain the glycolaldehyde product.
Glycolaldehyde may also be separated using extraction techniques. It can be difficult, however, to separate the glycolaldehyde into an easily isolated form without the simultaneous extraction of significant amounts of expensive rhodium catalyst or reaction solvent. Residual catalyst and solvent can interfere with the conversion of glycolaldehyde into further products such as ethylene glycol. In addition, it is sometimes necessary to recycle the organic phase from the extraction to reuse the solvent and catalyst in subsequent reactions. With extraction, it is also difficult to balance catalyst activity, glycolaldehyde extractability, and rhodium retention in the organic phase.
For example, lower molecular weight amide solvents such as, for example, dimethylformamide, dimethylacetamide, N,N-dimethylbutyramide, and N-methylpyrollidinone (NMP), give good catalyst activity, but are readily extracted into the aqueous phase along with some of the rhodium catalyst. Higher molecular weight amides, by contrast, can give better extraction performance but typically exhibit poor reaction rates. In view of the problems that remain in the art, there is a need for a process for the hydroformylation of formaldehyde to glycolaldehyde that can provide good reaction rates and selectivity to glycolaldehyde, allows for clean separation of the product and catalyst, and enables efficient recovery and recycle of the catalyst and solvent.